Certain process equipment in industrial installations, such as for the purpose of processing semiconductor wafers in the manufacture of circuit chips, requires the substantially uninterrupted flow of liquid chemicals to be drawn on at the proper time for performing specific functions in the processing equipment. For instance, certain stages of semiconductor wafer processing requires a supply of acid, such as hydrofluoric acid, for the purpose of etching oxides from the surfaces of the semiconductor wafers. The supply drums for such liquid chemicals are usually located in a protected room or building remote from the processing station, and arrangements are made in the facilities housing the supply drums for changing the drums when the chemical in a supply drum is depleted so that a new drum may be substituted for the empty one.
An important consideration in the supplying of liquid chemicals from such drums is to be sure that a continuous supply of the liquid chemical is available to the processing equipment. Accordingly, temporary storage facilities for the liquid chemical are provided between the supply drums and the processing equipment, but of course, the temporary storage facilities must be adequately supplied with the liquid chemical so that the supply to the processing equipment will be continuous.
Another important consideration in the supplying of liquid chemical from the source or supply drums, is the disposal of gas that may be drawn into the delivery system, particularly when the supply of liquid chemical in the supply drums is exhausted and air or gas is drawn into the delivery system. Gas may also be drawn into the delivery system by leakage. Usually a mechanical coupling is provided between supply tubes in such a supply drum, and flow lines which deliver liquid chemical to the processing facility. A representative coupling is illustrated in U.S. Pat. No. 5,108,015. Although such couplings make tight joints for liquid flow, gas leakage into the flowing liquid is not uncommon and must be accommodated.
Prior devices for generally the same purpose are illustrated in U.S. Pat. Nos. 3,787,829 and 4,601,409. The apparatus in both of these prior devices are subject to giving false readings of an empty drum when the supply drum may not actually be empty. In the '829 patent, false readings to indicate an empty drum may occur in the event leakage gases become entrained in the liquid chemical, and such gases collect in the float chamber. A substantial amount of temporary storage downstream of the pump in the '829 patent is required during recovery after an empty tank has been detected and a new supply drum has been substituted.
The '409 patent requires the use of a separate aspirator during recovery after an empty supply tank has been detected and a change to a different tank is effected. The '409 patent may also give false readings of an empty condition in the supply tank by virtue of gas leakage that may become entrained in the liquid being supplied and collection of such gas in the reservoir.